1. Field of the Invention
The present invention relates to devices and methods for electronic data entry. More particularly, the present invention relates to a device and method for remotely detecting the closure of a switch in a matrix of switches, as in a keypad, for data entry. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
A keypad, keyboard or other switch array for data entry to a host device typically takes one of four forms. Each of these forms has its own advantages and limitations.
Electronically simple, matrix keypads consist of multiple keys or other switches placed at the crossings of two parallel groups of conductors, which represent "rows" and "columns" of keys. When a switch is closed, it connects one row wire to one column wire. In a matrix keypad, all wires run separately to the host device. For large numbers of keys, or with significant distance between keypad and host, the number of wires running to the host device can require a large, expensive and cumbersome connecting cable.
A more sophisticated keypad uses various encoding schemes to send data back, for instance, by using active electronic circuitry to convert switch closures to ASCII equivalents for transmission to the host over a serial line. Such a keypad needs fewer connecting wires (typically four) than an unencoded matrix, but the fact that it contains active electronic components gives it three liabilities: it is more expensive than the simple matrix type, it requires a power source to drive the active components, and it is restricted to use in environments where these devices function can function reliably. Wet, very hot, very cold, or electrically noisy environments can place severe limitations on a keypad of this type.
Still more sophisticated is a wireless keypad, which requires no direct connection to the host but transmits data, for example, over a beam of infrared light. While offering great convenience, such a keypad has most of the liabilities of the encoded type, plus two others: it must contain batteries, solar cells, or some other independent power source, and it can be used only when there is a clear transmission path back to the host. It is, however, virtually immune to electrical noise. Wireless keypads are quite expensive and chiefly limited to "clean" environments such as the office or home.
All of the keypad types described above are "digital" in that they use signals or connections which are either "on" or "off". There is a fourth type of keypad, little-used at present, but quite inexpensive and requiring only a two-wire link back to the host. This fourth type transmits data by a variable analog current. Resistors or other current-limiting devices are connected through normally-open pushbutton switches between wires forming a current loop. With all switches open, the current in the loop is practically zero. Pressing each of the buttons allows a different level of current to flow. By measuring this current, comparator and gating circuitry in the host can determine which key was pressed. While analog current-loop transmission is inherently immune to most electrical noise, the available precision in analog circuitry (weighed against cost) normally limits the usefulness of this type of keypad to about eight or ten different keys and current levels.
Keypads and switch arrays have been the subject of several patents. Several of these use voltage divider networks to identify each switch by a different voltage level. See Ohtsuka, et al. (U.S. Pat. No. 4,872,008), Crumley, et al. (U.S. Pat. No. 4,429,301), and Strandt (U.S. Pat. No. 4,015,254).
Niki (U.S. Pat. No. 3,873,978) provides pairs of switching circuits responsive to the closure of a switch and that are biased to different voltage levels. Coupling rows and columns causes these circuits to change output logic levels and thereby supply signals characteristic of the particular row and column so coupled.
Osbome (U.S. Pat. No. 3,573,807), focuses on the generation of a different binary number when a switch in an array of switches is closed. He uses sets of transformers with different numbers of windings on the primary side to generate the numbers directly from the coupling of each switch closure.
Wilson (U.S. Pat. No. 4,303,907) describes the use of combinations of sourcing and sinking circuits and Field Effect Transistors (FETs) to design a matrix of switches. The FETs establish current mirrors, one mirror having a high impedance load; the other, a low load. Freeman (U.S. Pat. No. 4,906,993) has keys that have high impedance states plus input and output states to allow the number of switches to exceed the number of I/O lines.
No keypad currently available, however, seems to offer the combination of simplicity, low cost, simple connection to the host, freedom from active components, high electrical noise immunity, and potentially large numbers of keys.